Recent evidence suggests the importance of the innate immune system in both graft rejection and the induction of tolerance in solid organ transplantation. However, the specific cell types and molecules of the innate immune system involved in these processes have not been determined. We have begun to define natural killer (NK) cell activation in the context of solid organ transplantation. We have shown that NK cells comprise the majority of liver infiltrating cells early post-transplantation. These NK cells are functionally active expressing both granzyme B and FasL and producing substantial amounts of IFN-gamma. Studies have demonstrated extensive cross talk between NK cells and dendritic cells (DC) resulting in both the activation of NK cells and DC or the killing of DC. Elimination of antigen-bearing DC could prevent T cell priming and lead to tolerance. Although little is known about the precise nature of the NK cell interaction with DC, there is evidence to suggest that the NK cell receptor NKp30 is important. We have recently cloned and characterized the rat homologue of NKp30. We hypothesize that the innate immune system, specifically the activation of NK cells, influences graft outcome post transplantation. This hypothesis will be tested using a rat orthotopic liver transplant model to examine the role of NK cells during graft rejection and long term graft survival. In Specific Aim 1 we will determine the role of NK cells in both graft rejection and tolerance post-transplant. The effects of immunosuppressive drugs on modulation of NK cell infiltration and activity will be examined. We will examine the expression of NK cell activation receptors. The functional effects of activation receptor ligation in NK cells will be defined with a focus on cytokine production and cytotoxic activity. In Specific Aim 2 the functional outcomes of NK cell and DC interactions will be examined in a co-culture system and with graft-associated NK cells. Experiments will be performed to evaluate reciprocal activation of NK cells and DC and NK cell mediated-DC killing. In addition, the receptors involved, the molecular mechanisms, and the effect of immunosuppressive drugs on NK cell and DC interactions will be examined. The goal of Specific Aim 3 is to analyze the functional consequences of NK cell and DC interactions in an experimental model of liver transplantation. Our studies will specifically define the role of NK cells and potential interactions with DC in both allograft rejection and acceptance. Modulation of NK cell and DC interactions may promote specific allograft tolerance.